This invention relates to a control circuit for controlling and limiting via a semiconductor control switch the start-up current supplied to a load in two or more steps and in a manner so as to protect the semiconductor control switch from overload. More particularly, the present invention relates to a control circuit, for limiting the start-up current that flows through a semiconductor control switch connected in series circuit with an incandescent lamp or similar load device whose impedance exhibits a non-linear variation during the turn-on phase thereof.
Due to the non-linear impedance characteristic of an incandescent lamp or other similar non-linear impedance load, a turn-on or inrush surge current occurs which may be approximately ten times the steady state or normal operating current of the lamp or other load. If a semiconductor control device, such as a bipolar transistor or a field effect transistor (FET), is connected in series circuit with such a load across a pair of voltage supply terminals, damage to the semiconductor control device may occur at start-up unless some form of surge current protection is provided.
For energizing a lamp, one known and common constant current limiting technique is shown in FIG. 1 and includes a semiconductor power switch 1, such as an FET, connected in series circuit with a small sensing resistor 2 and the lamp 3 across a pair of voltage supply terminals. The voltage developed across the sensing resistor (R.sub.s), which is proportional to the current through the FET and the lamp, is applied to an input of a comparator 4 having a fixed input offset voltage (V.sub.os). The output of the comparator is coupled via a logic circuit 5 and a switch driver 6 to the gate or control electrode of the series connected power switch (FET). If the load current flowing through R.sub.s is large enough to trigger the comparator, a feedback signal is applied to the gate or control electrode of the FET via the logic circuit and the switch driver so as to turn-off the power switch. Assuming the feedback delay is small compared to the rise/fall time of the power switch (FET), the load current will be limited to a constant value (possibly with a small ripple component) equal to V.sub.os /R.sub.s.
FIG. 1A depicts the relationship of load current (I.sub.L) versus the output voltage (V.sub.o) across the lamp for the lamp control circuit described above. Also shown is the characteristic curve T.sub.c for the transistor power switch which defines the safe operating area (SOA) for the series connected FET power switch. The curve labelled TOC represents the turn-on characteristic for the incandescent lamp and shows the variation of the operation points during turn-on of the lamp as it heats up and its filament resistance increases continuously from a small resistance value R.sub.c when it is cold to a final steady state value R.sub.h after it has heated up.
In this type of prior art circuit, the current is initially limited to a value I.sub.L1. If, during the time that the power switch drives the lamp, part of the TOC of the lamp remains outside of the SOA of the power switch for a sufficient period of time, for example, from the time instant t.sub.1 to the time instant t.sub.2, damage to or destruction of the power switch may result due to overload thereof.
There are three common methods for protecting the power switch from overload during start-up of the lamp. The first is to reduce the current limit value from the value I.sub.L1 to a lower value I.sub.L2, where I.sub.L2 is now the maximum current allowed to flow and its value is chosen so that the current through the power switch never exceeds its power capability over the entire operating range of the load current and the load voltage. A disadvantage of this technique is that the small current flowing during start-up means less power is delivered to the load (lamp) and so the lamp will heat up and reach its normal operating resistance at a much slower rate.
A second switch protection method is to use a power switch with a much higher power handling capability such that it can safely handle the maximum lamp current which occurs when the filament is cold. This method has the obvious disadvantage that it requires a much larger power switch, a larger heat sink, etc, all of which increases the cost and size of the circuit.
A third way of protecting the power switch is to switch it on and off at a low duty cycle during the initial time period when the lamp resistance is low thereby to reduce the power dissipation in the switch. One disadvantage of this approach is that the power pulses generated produce undesirable electromagnetic interference (EMI).
European patent application, EPA No. 0,285,4l7, published Oct. 5, 1988 discloses a solid state switch for limiting the flow of start-up current to an incandescent lamp. The control circuitry in this device initially allows a relatively low constant current to flow through the lamp and a series connected FET switch and then, automatically, after the lamp resistance reaches a preselected level, it is allowed to draw a significantly higher current. The size and cost of the power FET is reduced significantly because the magnitude of the current spike generated at turn-on of the lamp is reduced. A disadvantage of this circuit is that effectively it provides only one current limit. When it switches over automatically, a fairly large current spike nevertheless is still allowed to flow, albeit lower than would otherwise occur in the absence of the invention described therein. Furthermore, that device uses operational amplifiers to bias the current delivered to the lamp, rather than comparators operative to clamp the current to a certain value. As a result, the EPA apparatus requires frequency compensation and is therefore harder to implement in an integrated circuit.
The foregoing problems related to the operation of a lamp load are compounded in the case of an automotive environment which requires special techniques to drive an automotive lam load.